1. Field of the Invention
The present invention relates to driver circuits and more particularly to controlling the output impedance of driver circuits.
2. Description of the Related Art
In computer and information processing systems, various integrated circuit chips must communicate digitally with each other over common buses. The signal frequency at which this communication occurs can limit the performance of the overall system. Thus the higher the communication frequency, the better. The maximum frequency at which a system communicates is a function not only of the time that it takes for the electromagnetic wavefronts to propagate on the bus from one chip to another, but also of the time required for the signals to settle to levels that can be reliably recognized at the receiving bus nodes as being high or low, referred to as the settling time.
There are several factors which affect the settling time of a signal. For example, ringing due to reflections from impedance mismatches within the bus system is a factor which affects the settling time of the signal. The voltage level of the launched signal relative to the overall signal swing is another factor which affects the settling time of the signal. The effectiveness of the termination of the bus is another factor which affects the settling time of the signal. Ringing, the relative voltage level and termination of the bus are controllable by controlling the output impedance of the driver. However, controlling the output impedance of an output driver can be challenging.
More specifically, because drivers are typically constructed with transistors, and often CMOS transistors, the operating characteristics of the transistors affect the output impedance of the driver. The operating characteristics of transistors can change under a variety of circumstances. For example, these operating characteristics change with variations in operating conditions such as junction temperature and supply voltage levels. The operating characteristics may also change with variations in manufacturing process.
If inadequate compensation is made for these variations, the output impedance of the driver may vary substantially within a particular chip from time to time as well as from chip to chip.